Search

Copyright of Boletín de la Sociedad Geológica Mexicana is the property of Sociedad Geologica Mexicana and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The Zn-Pb-Cu(-Ag±Au) San Martín deposit in northwestern Zacatecas is one of the most economically important and biggest skarns in Mexico. Mineral associations in this deposit belong to the sulfide skarn type (with rather "classical" prograde and retrograde zones) and contain peripheral subepithermal to epithermal veins. Re-Os ages were obtained for two molybdenite samples from deep Cu-Zn-Pb±Mo±Bi±Ag retrograde mineralization in these deposits, of 43.7 ± 0.3 and 44.0 ± 0.2 Ma, which correspond essentially to the same age. These are 1 to 3 Myr younger than preexisting K-Ar ages for the granitic stock that generated these mineral deposits. Prograde mineral associations are hereby interpreted to have been directly associated with this intrusion. Therefore, we may calculate a simple decrease rate in temperatures of hydrothermal fluids between ~100° and > 300 °C per million years from prograde to retrograde skarn associations. Subepithermal and epithermal veins, however, remain undated. [ABSTRACT FROM AUTHOR]

Abstract: The Zn–Pb–Ag skarns of Zacatepec are located in the eastern Sierra Madre del Sur, southern Mexico. They are hosted by the Cuicateco Terrane, a composite unit made up of deformed Jurassic and Cretaceous volcanic and sedimentary rocks deposited in an arc environment that also hosts several precious- and base-metal epithermal deposits and Cu–Mo porphyries. The mineralization occurs as irregular masses along the contact between Cretaceous limestones and Tertiary porphyritic dacites. Over widths of few decimeters, an epidote-rich (Ep44–97Czo01–55Pie00–03) endoskarn is developed. For the most part, the mineralization consists in skarns of calcic garnet (Ad62–22Gr78–38) with disseminated hematite. Calcic garnets show chemical oscillatory zonation. Skarns rich in calcic clinopyroxenes (Di28–54Hd22–55Jo07–32) locally occur, farther from the porphyry contact. As a result of a retrograde event, the above assemblages are locally silicified, and ore minerals (sphalerite, galena, pyrite, chalcopyrite and minor scheelite) and chamosite deposited. The iron contents in sphalerite range between 5.0 and 9.4mol% FeS. An assemblage of sulfosalts, consisting of vikingite Ag5Pb8Bi13S30 and an Ag–Bi–S phase probably of the aikinite group, occurs. Locally, magnesian skarns develop and are composed of almost pure diopside (Di98–100Hd00–01Jo00), clinochlore, chrysotile, and vesuvianite notoriously rich in Ti (6.48 to 7.93wt.% TiO2). The magnesian skarns yield the highest metal grades, up to 595g/t of Ag, 9.94% Pb and 3.27% Zn. After weathering of the ores, botryoidal crusts of supergene minerals as smithsonite, rosasite (Cu,Zn)2(CO3)(OH)2 and hemimorphite developed. Microthermometric studies of fluid inclusions were carried out in calcite, calcic garnet, quartz and sphalerite. Homogenization to the liquid phase (Th) occurred between 160°C and 470°C. Salinities were calculated assuming an H2O–NaCl system and range from 10.5 to 19.8wt.% NaCl eq. The temperature of formation of the prograde skarn, calculated from microthermometric data with an assumed pressure of 500bar, ranges between 470°C and 510°C. A slight increase in ƒO2 caused the formation of garnet after clinopyroxene. The formation of the retrograde assemblage implies a decrease in temperature and, initially, in salinity, coupled with a great increase in ƒO2. These changes can be explained by mixing with cooler, oxidizing and dilute meteoric water. Subsequently, a progressive increase in fluid salinity as Th dropped from 300°C suggests that boiling took place triggering sulfide precipitation. [Copyright &y& Elsevier]

Copyright of Boletín de la Sociedad Geológica Mexicana is the property of Sociedad Geologica Mexicana and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The Mazatán barite deposits, Sonora, NW Mexico, represent an outstanding example of Paleozoic bedded barite, a poorly understood type of mineral deposit of major economic interest. The deposits of this type commonly occur hosted by shales and are characterized by the lack of base-metal sulfide mineralization, in contrast to classic sedimentary-exhalative (or SEDEX) deposits. A throughout study of the Mazatán barite deposits, based on petrography, fluid inclusions analyses and isotope geochemistry, confirmed the link between bedded barite and fossil hydrocarbon seeps, thereby leaving hydrothermal vent-related processes ruled out. Hence, modern cold seeps in continental margins would account for the geological setting and genetic aspects of this type of deposit. The largest barite bodies of Mazatán are hosted within an Upper Carboniferous flysch succession, which formed part of an accretionary wedge related to the subduction of the Rheic Ocean beneath Gondwana. As well, a few barite occurrences are hosted in Upper Devonian, pre-orogenic turbidites. A variety of mineralized structures is displayed by barite, including: septaria nodules, enterolitic structures, rosettes and debris-flow conglomerates. Barite is accompanied by chalcedony, pyrite (framboids) and berthierine. Gas-rich fluid inclusions in barite were analyzed by micro-Raman spectroscopy and methane was identified, suggesting the occurrence of light hydrocarbons in the environment within which barite precipitated. 13C-depleted carbonates (δ13C: −24.3 to −18.7‰) were found in the barite deposits; they likely formed through anaerobic oxidation of methane coupled to sulfate reduction. Besides, these carbonates yield negative δ18O values (−11.9 to −5.2‰) reflecting the isotopic composition of Devonian–Carboniferous seawater; alternatively, this 18O-depletion could reflect late diagenetic processes. Methane-derived carbonates occur at modern hydrocarbon seeps and have been reported from Mesozoic, Cenozoic and even Paleozoic seep sediments, but they have never before been described in Paleozoic bedded barite deposits. δ34S values of barite vary from +17.6 to +64.1‰, with the lowest values overlapping the range for coeval seawater sulfate; this distribution indicates a process of sulfate reduction. Barite precipitation can be explained by mixing methane- and barium-rich fluids with pore-water (seawater) containing sulfate residual from microbial reduction. Two analyses from barite gave an 87Sr/86Sr within and slightly above the range for seawater at the time of deposition, with 0.708130 and 0.708588, which would preclude the involvement of hydrothermal fluids in the mineralization process. [ABSTRACT FROM AUTHOR]

Abstract: The Francisco I. Madero deposit, central Mexico, occurs in the Mesozoic Guerrero Terrane, which hosts many ore deposits, both Cretaceous (volcanogenic massive sulfides) and Tertiary (epithermal and skarn deposits). It is hosted by a 600 m-thick calcareous-pelitic unit, of Lower Cretaceous age, crosscut by porphyritic dikes that strike NW–SE. A thick felsic volcanic Tertiary sequence, consisting of andesites and rhyolitic ignimbrites, unconformably overlies the Cretaceous series. At the base, the mineralization consists of several mantos developed within calcareous beds. They are dominantly composed of sphalerite, pyrrhotite and pyrite with minor chalcopyrite, arsenopyrite and galena. At the top of the orebody, there are calcic skarns formed through prograde and retrograde stages. The resulting mineral assemblages are rich in manganoan hedenbergite (Hd75–28Di40–4Jh40–20), andraditic garnets (Adr100–62Grs38–0), epidote (Ep95–36Czo60–5Pie8–0), chamosite, calcite and quartz. The temperature of ore deposition, estimated by chlorite and arsenopyrite geothermometry, ranges from 243° to 277 °C and from 300° to 340 °C, respectively. The pressure estimated from sphalerite geobarometry averages 2.1 kbar. This value corresponds to a moderately deep skarn and agrees with the high Cu content of the deposit. Paragenesis, P–T conditions and geological characteristics are compatible with a distal, dike-related, Zn skarn deposit. Its style of mineralization is similar to that of many high-temperature carbonate replacement skarn deposits in the Southern Cordillera. [Copyright &y& Elsevier]